Method and apparatus for the production of high tenacity polyolefin sheet

ABSTRACT

A process for the production of virtually full density polyolefin suitable for further processing by drawing to form a high tenacity, highly oriented polyolefin sheet comprising: a) feeding a metered amount of polyolefin powder into the nip between two heated calender rolls initially set at a gap smaller than the size of the smallest polyolefin powder particle and at a temperature above the melting point of the powder; b) rolling the powder through the nip under these conditions until a coherent sheet of polyolefin is produced: and c) once a coherent sheet of polyolefin exits the nip lowering the temperature in the nip to a temperature below the melting point of the polyolefin powder and increasing the gap to a desired level above the thickness of the largest powder particle. Apparatus for the performance of such a process is also described.

This application is a divisional of U.S. patent application Ser. No.12/080,197 filed Apr. 1, 2008 and copending herewith.

FIELD OF THE INVENTION

The present invention relates to ultra high molecular weightpolyethylene (UHMWPE) and other high molecular weight polyolefinmaterials useful for ballistic applications and more particularly to anovel and highly economical process for their production.

BACKGROUND OF THE INVENTION

The processing of ultra high molecular weight polyethylene (UHMWPE),i.e. polyethylene having a molecular weight in excess of about 2million, is known in the polymer arts to be extremely difficult.Products made from such materials are, however, very strong, tough anddurable.

In the following series of U.S. patents filed by Kobayashi et al andassigned to Nippon Oil Co., Ltd. a number of inventions related to thefabrication of fibers and films of polyolefins generally and UHMWPEspecifically, are described: U.S. Pat. Nos. 4,996,011, 5,002,714,5,091,133, 5,106,555, 5,200,129, and 5,578,373. The processes describedin these patents generally describe the continuous production of highstrength and high modulus polyolefin films by feeding polyolefin powderbetween a combination of endless belts disposed in an up and downopposing relationship, compression molding the polyolefin powder at atemperature below its melting point between the endless belts and thenrolling and stretching the resultant compression molded polyolefin intoan oriented film. As compression molded, the sheet is relatively friablethus requiring the subsequent calendering or drawing operations toprovide an oriented film that exhibits very good strength and durabilityproperties. In fact, the strength of such materials produced by theseprocesses can be 3 times that of steel on a weight basis and theyexhibit very low creep.

Enhanced processes for the production of such materials have also beendescribed in the following U.S. Pat. No. 7,348,053 and U.S. patentapplication Ser. No. 11/217,279 filed Sep. 1, 2005.

A common element of all of these prior art processes is that theyrequire compaction of an UHMWPE powder as the initial step in theproduction process. Until now, it has been the thinking of the UHMWPEmanufacturing community that such powder compaction was necessary inorder to place the material in a form that it could be subsequentlyrolled and drawn as described in the referenced prior art. Stateddifferently, it has been the thinking that in order to produce theproduct in a process involving the subsequent rolling and drawing stepsto obtain the orientation required for the production of ballisticallyuseful UHMWPE, the powder had to first be placed in the form of a sheetthat demonstrated sufficient tenacity to be successfully processed insuch subsequent rolling and drawing processes. In the prior art, such aform was obtained by compacting the powder into a relatively friablesheet that could be introduced into the rolling operation for subsequentprocessing.

The performance of this compaction process step, particularly in theproduction of UHMWPE sheets wider than 1-2 inches in width, requires theuse of relatively massive, quite complex and very expensive equipment(measured in the millions of dollars for installed such equipment). Suchequipment thus requires high levels of capital expenditures forinstallation and due to its complexity ongoing high operating andmaintenance expenses.

U.S. Pat. No. 4,436,682 to Knopp, issued Mar. 13, 1984 describes aprocess for compacting polymer powders into fully dense products.According to this patent, a polymer powder is fed from a hopper into thenip between two rolls, compacted therein at a temperature below themelting point of the polymer powder and withdrawn from the nip undertension to form a “fully dense” polymer sheet. According to Knopp, whenhis process is applied to an UHMWPE powder, the resulting sheet has adensity of about 0.82 g/cc which he designates as “substantially fullydense”. It is well known that the density of UHMWPE is on the order ofabove 0.945 g/cc. Hence, the product of Knopp's process is hardly “fullydense” and is unsuited to further processing by calendering or drawing,since it will tear or break when subjected to such processes.

It would thus be of great benefit to the producer of such UHMWPEmaterials, particularly in widths greater than a couple of inches, if amuch simpler, smaller and less expensive first process step could besubstituted for the powder compaction step, without negatively affectingthe either the product thus produced or significantly affecting thekinetics of the process, i.e. it did not, for example, slow productionto an uneconomical rate.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide anenhanced process for the production of UHMWPE sheet that eliminates theneed for the previously described compaction step and uses a much morecost effective and simpler process for the production of a high tenacityUHMWPE sheet that can undergo subsequent processing by drawing.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process for theproduction of virtually full density polyolefin suitable for furtherprocessing by drawing to form a high tenacity, highly orientedpolyolefin sheet comprising: a) feeding a metered amount of polyolefinpowder into the nip between two heated calender rolls initially set at agap smaller than the size of the smallest polyolefin powder particle andat a temperature above the melting point of the powder; b) rolling thepowder through the nip under these conditions until a coherent sheet ofpolyolefin is produced: and c) once a coherent sheet of polyolefin exitsthe nip lowering the temperature in the nip to a temperature below themelting point of the polyolefin powder and increasing the gap to adesired level above the thickness of the largest powder particle. Such aprocess not only eliminates the need for a separate and costlycompaction step, but yields a coherent polyolefin sheet that is readyfor drawing in accordance with prior art processes for the production ofa high tenacity, highly oriented polyolefin sheet having a high heat offusion. According to a preferred embodiment of the present invention,the polyolefin of choice is ultra high molecular weight polyethylene(UHMWPE).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the production processes of theprior art.

FIG. 2 is a schematic diagram of a preferred embodiment of the apparatusused to implement the drawing portion of the preferred process of thepresent invention.

FIG. 3 is a schematic side view of the apparatus used to producecoherent UHMWPE sheet in accordance with the present invention.

DETAILED DESCRIPTION

In the description that follows, operating parameters, materialproperties etc. are presented in the context of those for ultra highmolecular weight polyethylene (UHMWPE), but it will be readilyunderstood by the skilled artisan in the polymer field that theinvention described herein is readily applicable to other polyolefinpolymers such as high molecular weight polypropylene through thejudicious selection of materials and process conditions appropriate forthese other polyolefin materials.

The term “tape” as used herein refers to products having widths on theorder of or greater than about ½ inch and preferably greater than 1inch. The term “fiber” as used herein is meant to define a “narrow”tape, i.e. an element narrower than about ½ inch. The term “slit filmfiber” refers specifically to a “fiber” or narrow tape made inaccordance with the present invention that exhibits a generallyrectangular cross-section and smooth, i.e. non-serrated or ragged edges.The terms “sheet” and “film” as used herein is meant to refer to thinsections of the materials of the present invention in widths up to andexceeding 160 inches in width as could be produced in large commercialequipment specifically designed for production in such widths. Accordingto a preferred embodiment, such sheets and tapes have a generallyrectangular cross-section and smooth edges. Hence, the fundamentaldifference between a “tape”, a “slit film fiber”, a “fiber”, a “film”and a “sheet” as used to describe the products of the processesdescribed herein relates to the width thereof and is generallyindependent of the thickness thereof.

Referring now to FIG. 1, the processes described in the prior art anddepicted schematically in FIG. 1 comprised the continuous production ofhigh strength and high modulus polyolefin films by feeding polyolefinpowder between a combination of endless belts disposed in an up and downopposing relationship, compacting the polyolefin powder at a temperaturebelow its melting point between the endless belts and then rolling andstretching the resultant compression molded polyolefin into an orientedfilm. To the extent of their relevance to the modified processesdescribed herein, the aforementioned prior art descriptions contained inU.S. Pat. Nos. 4,996,011, 5,002,714, 5,091,133, 5,106,555, 5,200,129,and 5,578,373 are incorporated herein by reference in their entirety.

A major difference between the processes of the prior art and those ofthe present invention is that the present invention obviates the needfor the compaction step and its related high cost entirely. Thus, themethod described herein begins with heated polyolefin powder introducedas described hereinafter directly into a pair of heated, counterrotating calender rolls under very specific temperature and gapconditions to produce a coherent polyolefin sheet suitable forsubsequent further drawing to orient the polyolefin and to produce aballistically useful high tenacity, highly oriented polymeric material.

According to a preferred embodiment of the present invention, thepolyolefin processed in accordance with the process of the presentinvention is an UHMWPE that exhibits high crystallinity (above about 80%as determined by differential scanning calorimetry), a heat of fusionequal to or greater than 220 joules/gram and low levels of entanglement.Thus, it is preferred that the input starting material UHMWPE possessthe degree of crystallinity and heat of fusion and meet the lowentanglement requirements stated above. Such commercially availablematerials as Ticona X-168 from Ticona Engineering Polymers, 2600 UpdikeRoad, Auburn Hills Mich. 48236 and type 1900 CM from Basell Corp. 2801Centerville Road, Wilmington, Del. 19808 are useful in the successfulpractice of the present invention.

Referring now to the accompanying drawings, as depicted in FIG. 3, theinitial step in the process of the present invention utilizes a directroll apparatus 10 comprising a polymer powder hopper 12 that feeds ametered amount of polymer powder 14 into a vibratory chute 16 via ametering device 18 and thence to a containment plate 20. At containmentplate 20 the powder is introduced into the gap 22 between two counterrotating heated calender rolls 24 and 24A rotating in the directionsshown by arrows 26 and 26A. A heater 27 preferably an infrared heater,imparts heat to powder 14 as described more fully below. Heater 27 ispreferably located from about 2 to about 8 inches above powder 14 invibratory chute 16 and set at a temperature of between about 160 and220° F. These distances and temperatures will, of course, be variabledepending upon the particular polymer powder 14 being processed and thetype of heater used, but have been found suitable for the processing ofthe preferred UHMWPE. As powder 14 cascades down vibratory chute 16 ontocontainment plate 20 it builds to a point where it is drawn into the gapor nip 22.

The successful practice of the present invention requires that at thestart of the direct roll process, gap 22 be set narrower than the sizeof the smallest individual polymer powder particle, for example at about50μ. Gap 22 may, of course, be widened if the minimum particle size ofpolymer powder 14 is greater than 50μ. Similarly, at start up of thedirect rolling process described herein, heated calender rolls 24 and24A are heated to a temperature above the melting point of polymerpowder 14. While this melting point will be dependent upon theparticular material being processed, in the case of the preferred UHMWPEstarting materials described elsewhere herein this initial temperatureis about 149° C. or about 3° C. above the melting point of the preferredUHMWPE. Lower temperatures could, of course, course be appropriate forlower melting polyolefin materials. At this point, rolling of powder 14is initiated. As soon as a coherent sheet of polymer 28 begins to emergefrom gap 22 the temperature of calender rolls 24 and 24A is reduced tobelow the melting point of polymer powder 14 and gap 22 is increased tothat desired for the final product thickness for coherent sheet 28. Asused herein, the term “coherent sheet” is meant to define a polymersheet that is suitable for further processing by drawing withouttearing, ripping or otherwise becoming unusable in such additionalprocessing. For all practical purposes, such a sheet will be virtuallyfully dense such as in the case of the preferred UHMWPE materialsdescribed herein having a density above about 0.945 g/cc. For thepreferred UHMWPE powders 14 described elsewhere herein the operatingtemperature (the temperature after formation of a coherent sheet 28 isin the range of from about 136 to about 144° C. and preferably betweenabout 139 and about 141° C., and the operating gap is on the order of100μ and 230μ and preferably at about 140μ. It should be noted that theinitial and operating temperatures recited herein are not necessarilyset points for the polymer powder/sheet in nip 22, but rather surfacetemperatures of heated calender rolls 24 and 24A.

While the operating speed of the apparatus just described will vary withthe particular polyolefin being processed, using the preferred UHMWPEmaterials described above, start up roll speeds of between about 1.9 toabout 4.0 meters per minute have been found acceptable. Steady stateoperation of the apparatus is generally within the range of betweenabout 2.0 and about 12.0 meters per minute. It should be noted thatthese operating speeds are based primarily on ones ability to take upcoherent sheet 28 and the size of hated calendar rolls 24 and 24A, sincelarger rolls will generally tend to increase the surface in contact withthe polymer in nip 22. Thus, if downstream operations or take upapparatus are capable of faster speeds, or larger diameter rolls areused, higher operating speeds for the direct roll process just describedare possible.

The product of the just described process is a virtually full dense andtranslucent UHMWPE sheet, i.e. an UHMWPE sheet having a density of about0.95 to about 0.98 g/cc.

While the apparatus used to practice the process of the presentinvention is depicted herein as vertically oriented, the process willoperate equally well in a horizontal configuration, i.e. with thepolymer powder being fed to gap 22 between two horizontally parallelcalender rolls 24 and 24A. In this orientation, powder 14 is meteredfrom a heated hopper located above horizontally parallel calender rolls24 and 24A so that powder 14 is fed from above into gap 22 and theproduct sheet 28 is drawn from below gap 22. All other operatingprocedures, i.e. temperature control and gap setting variations remainthe same.

While not critical to the successful practice of the present invention,and clearly variable depending upon the particular polyolefin beingprocessed, roll surface roughnesses of from about 4 to 8 RMS (Root MeanSquare) have been found suitable for the processing of the preferredUHMWPE materials described herein.

Post-processing of coherent sheet 28 to obtain a highly useful UHMWPEballistic sheet, film, tape or fiber is performed in much the samefashion as and in apparatus similar to that described in issued U.S.Pat. No. 7,348,053, issued Mar. 25, 2008, i.e. by drawing coherent sheet28 which are referred to and incorporated herein in their entirety.

Referring now to FIG. 2, the drawing apparatus 40 utilized to achievethe thickness reductions of the coherent sheet produced as justdescribed that result in production of the preferred UHMWPE products ofthe present invention 10 comprises:

a payoff 42, a godet stand 44 including heated godet rolls 46 (to annealthe product) and nip rolls 48 for establishing and maintaining tensionin the line, a first draw zone 50, a first in-line tension sensor 52, asecond godet stand 54, a second draw stand 56, a second in-line tensionsensor 58, and unheated take-up rolls 68. As seen from FIG. 1, the inputor starting material of this process is generally the thick, compactedand rolled but unoriented product of the compaction step of the priorart production process. According to the preferred process of thepresent invention, the input or starting material in thedrawing/calendaring process steps described below is, of course,coherent sheet 28 that emerges from gap 22 in the process describedabove.

Each of the elements of the apparatus just described and utilized in thesuccessful practice of the present invention are well known in the filmand fiber drawing arts as is their combination in a line of the typejust described. Consequently, no detailed description of such a line isrequired or will be made herein and the reader is referred to thenumerous design manuals and descriptions of such apparatus commonlyavailable in the art.

Maintaining a constant tension of between about 0.5 and about 5g/denier, and preferably between about 0.8 and 3 g/denier during drawingis also important to the production of product having the required“thinness” and other enhanced properties specified herein. The term“denier” as used herein is defined as the weight in grams of 9000 metersof the product film, tape, sheet or fiber. At tension levels below 0.5g/denier no significant drawing or reduction will be obtained while attension levels above about 5 g/denier the material will tend toseparate. In the case of drawing, tension is a function of the feedpolymer and can vary broadly depending thereon and the ranges justspecified refer to those found useful with particular preferred UHMWPEcommercial starting materials.

Total reductions achieved during drawing and calendaring will generallybe between about 50:1 and about 170:1 or more depending again upon theinput raw material and the end use to which the product is to beapplied. Such total drawing and calendaring is computed as the multipleof each of the individual reductions achieved by each of the combinedprocess steps.

According to a highly preferred embodiment of the present invention,drawing is performed in line with direct rolling as describedhereinabove. In such a continuous process, calender rolls 24 and 24Abecome payoff 42 of drawing apparatus 40. Such an arrangement provides ahighly efficient method for practicing the novel production process ofthe present invention.

After thickness reduction by drawing in the apparatus shown in FIG. 2according to the processing parameters just described, the UHMWPE films,sheets, fibers or tapes thus produced exhibit heats of fusion at orabove about 243 joules/gram, tenacities in the range of from about 18and 20 g/d, tensile moduli between about 1200 and about 1800 g/d andelongations in the range of from about 1.6 to about 2.0 percent.

There has thus been described a novel process for the production ofcoherent polyolefin, preferably UHMWPE, sheet and high tenacity, highlyoriented polyolefin, preferably UHMWPE, sheet, film, tape or fiber thateliminates the need for the prior art compaction step which, until thedevelopment described herein, was considered necessary for thesuccessful production of such materials.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

1) Apparatus for the production of a coherent virtually full densitypolyolefin sheet suitable for further processing by drawing to produce ahigh tenacity, highly oriented film, tape, fiber or sheet from apolyolefin powder comprising: a) a hopper including a metering device;b) a vibratory chute; c) a containment plate; d) a pair of counterrotating heated calender rolls defining a nip there between; and e) aheater above the vibratory chute; wherein powder is delivered from thehopper to the vibratory chute via the metering device where the powderis preheated by the heater and thence to the containment plate where itaccumulates and is introduced into the nip. 2) The apparatus of claim 1wherein the counter rotating heated calender rolls are stackedvertically. 3) The apparatus of claim 1 wherein said nip includes a gapbetween said pair of counter rotating heated calender rolls; and saidgap is capable of being set at less than 50μ during startup of saidapparatus and said gap is set between 100μ and 230μ for operation ofsaid apparatus. 4) The apparatus of claim 1 further including a drawingapparatus in line with said apparatus for the production of a coherentvirtually full density polyolefin sheet, said drawing apparatus fordrawing said polyolefin sheet to form a high tenacity, highly orientedpolyolefin sheet. 5) The apparatus of claim 4 wherein said drawingapparatus includes one or more drawing stations for drawing saidcoherent virtually full density polyolefin sheet. 6) The apparatus ofclaim 5 wherein said drawing apparatus includes one or more tensioncontrollers for maintaining a constant tension on said sheet duringdrawing of between 0.5 and 5 grams/denier. 7) The apparatus of claim 1wherein said counter rotating heated calender rolls include a rollsurface roughness of between 4 and 8 RMS. 8) The apparatus of claim 1wherein said counter rotating heated calender rolls are capable of rollspeeds of from 1.9 to 12.0 meters/minute.